Hydraulic presses generally comprise a pair of platens at least one of which is displaceable toward the other in a press frame or structure (see my U.S. Pat. No. 4,123,929) and one of which may form a bed of the press while the other is a platen mounted on the head.
Generally the hydraulic actuation system for such a press comprises a number of hydraulically supplied piston and cylinder arrangements, hereinafter referred to generally as hydraulic cylinders, which are disposed between a support member and the movable platen and are extensible and contractable by the hydraulic system to close or open the press, i.e. shift the movable platen toward or away from the opposite platen.
It is also known to provide, in such systems, a variable-displacement pump i.e. a pump whose displacement per revolution can be varied, to supply the hydraulic cylinder or hydraulic cylinders and to provide an energy storage system for delivering the energy required during the operating stroke of the press. This energy storage system can include a flywheel which can be connected to the pump shaft. Presses of this type can be single-level or multilevel presses, e.g. for the production of pressed board or for laminating pressed board or the formation of similar composites, and even deep drawing presses.
In a press system of this type (see German Pat. No. 2,349,351), the variable-displacement pumps can be operated by an electric motor and the return flow of the hydraulic medium can also be used to accelerate the flywheel and thereby charge the latter with energy. In this case, the flow through the pump is such that the pump can be constituted as a motor (see German patent document DE-AS No. 20 22 812).
While these systems are effective once they are in full operation, problems are encountered when startup of the press is necessary and an initial or minimum energy charge must be imparted to the flywheel. When the flywheel is fixed to the pump shaft, the high inertia required to drive the shaft means with an electric motor coupled thereto must likewise overcome this inertia and must develop extremely high starting torques. This necessitates a motor of larger capacity and current drain than is desirable, as well as associated devices to enable the motor to be effectively coupled to the pump shaft such as running clutches and other mechanical and electrical elements which all must have comparatively large dimensions and capacities.
Not only are the capital costs engendered by this relationship of the check motor to the high inertia flywheel and variable-displacement pump, substantial, but operating and maintenance costs are considerable as well.